Charles, the 9th Earl Spencer, 59, regularly shares updates about his ancestral home, Althorp House, from interior decorations to historical discoveries, but he was feeling reflective about his childhood recently.
Next to a photo of the majestic exterior bathed in soft sunlight, Charles revealed the appearance of the Northamptonshire Grade I listed stately home left him "overwhelmed by the formality" while growing up with his older sisters Princess Diana, Sarah and Jane.
"The classical lines of Althorp from the south-facing front courtyard. Built in 1508 as a red brick Tudor structure, @althorphouse was remodelled in the 1780s by the architect Henry Holland who clad it in 'mathematical tiles' - inch-thick grey-white limestone fronting that has dappled over the past 250 years.
"As a child visiting my grandparents here, I felt overwhelmed by the formality of the place - but now I love its essential Englishness," he wrote.
Princess Diana moved into the property at age 14, and it is where she first met her future husband King Charles in 1977 while he was dating her sister Sarah.
The late Princess was buried at Althorp following her death in August 1997 in a car accident in Paris. The initial plan was for Diana to be laid to rest in the family vault at the local church, but with concerns for security and privacy, Charles decided on an island at the centre of the family home's oval lake.
Prince Harry opened up about visiting his mother's private grave with his wife Meghan Markle in his book Spare. "The bridge had been removed, to give my mother privacy, to keep intruders away," he wrote.
However, the house is open to visitors in the summer months, with areas such as the 115-foot-long Picture Gallery and Wootton Hall, the entrance hall accessible. Charles explained the opening dates were a "strange coincidence", as they are meaningful dates for his late sister.
"With these big houses and their contents, you do an agreement with the government as to how many days to be open," he said on model Twiggy's podcast Tea with Twiggy. "We agreed to be open for July and August, so we open on the first of July, which is Diana's birthday, and we close on 31 August, which is the day she died."
He added: "That was agreed five years before she passed away, so strange."
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